Photoperiodic species, such as the Siberian hamster, must initiate changes in physiology and behavior (e.g., molt, gonadal regression and recrudescence, and torpor) well in advance of actual changes in the environment, because many of these physiological processes take several weeks to accomplish. The primary cue used to time these seasonal changes is daylength, which is transduced as the duration of the nocturnal melatonin signal. However, other factors must modify this photoperiodic information, as field studies have shown that the onset of seasonal changes may vary by up to four weeks from one year to the next. Ambient temperature has been implicated as a possible factor in modifying photoperiodic information, but the relation between temperature and photoperiod cues has not been well characterized. The goals of the proposed research are: (1) to investigate the extent to which ambient temperature modifies photoperiodically-induced seasonal changes in physiology and behavior in the Siberian hamster, (2) to determine whether the effects of ambient temperature on the photoperiodic system are mediated by changes in nocturnal melatonin production or modifications in the responsiveness of target tissues to melatonin, and (3) to investigate whether lowered body temperature during torpor bouts inhibits melatonin production during torpor or effects subsequent euthermic melatonin production.